Priority Queues

An endpoint is a collection of handlers that process a specific message type. Some messages might be considered more important than others. As queues generally work according to the First In, First Out (FIFO) principle, those important messages have to wait until all messages that arrived earlier, are processed first.

This behavior can potentially awaken the need for those important messages to 'jump the queue' and bypass all those other messages. This is regularly solved using priority queues. A special queue next to the regular queue that holds messages that will be processed before messages in the regular queue.

This article tries to explain a different type of solution.

Different business process

Often the need to process messages quicker and have the need for priority messages comes from a business standpoint. It is often more than message priority. A different type of process could exist for the priority message. Or a different kind of fee applies or even larger differences in business logic exist than for regular messages. Depending on the domain there can be many deviations from the regular business processes.

The result is that the design of a component is about more than message priority. There could be differences in code or even in design. At the very least it's usually more than just the priority of the message. It's likely the design and code should be separated completely from the regular business process.

Different operational considerations

Another aspect of priority messages can be a different Service Level Agreement (SLA) than with regular messages. The SLA might be breached if the message isn't processed in a specific amount of time, which is far more strict than with regular messages.

Other decisions could be to use a different datastore, concurrency mode or implement a different caching strategy. Or even decide for a different type of datastore or different hardware. It should also be possible to scale out differently, compared to the regular business process. Have different metrics, monitoring, etc.

Low coupling

Endpoints should be designed to be autonomous and be able to do their work without having knowledge of how other endpoints work. If an endpoint is required to decide which messages should have a higher priority, it requires knowledge that the receiving endpoint(s) are responsible for. This requires a tighter coupling than might be desirable. If the Single Responsibility Principle is valued, it is recommended not to introduce more responsibilities into a sender than necessary. As the result would be that if the logic changes to decide if a message has priority, the sender needs to be changed as well.

Possible solutions

A solution is to place the responsibility of deciding which message is a priority message with the sender. As mentioned, this introduces tight coupling between sender and receivers.

Another option could be using publish/subscribe where both the regular- and high priority endpoint both subscribe to a publishing endpoint.

In a series of blog posts about priority queues, Dennis van der Stelt goes into more details of both the problem and possible solutions and their trade-offs. He provides a third solution that is not discussed here, that involves a different view on responsibilities and deployment using concepts from the 4+1 architectural view model.

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